An increasing number of portable electronic products are available today which are powered by a battery pack. These products include such things as cellular telephones, portable radios, pagers and voice recorders which are conveniently mobile and operate using rechargeable batteries. Many different battery chemistries have been used for many years which meet the need for recharging capability. Probably the most popular chemistries include nickel cadmium and nickel metal hydride. A relatively new chemistry, however, generally referred to as lithium ion, but may include similar lithium secondary cell chemistries such as lithium polymer, enables a cell to be recharged while offering many advantages over other types of rechargeable cells. These benefits primarily are directed to low weight and overall size with a high energy density. One unique factor to be considered when using a lithium ion cell is its charging scheme. A lithium ion cell is not charged in the same manner as cells utilizing a nickel chemistry.
Nickel-cadmium and nickel metal hydride cells are typically charged using a rapid charge by applying a constant current until a certain event occurs. This event may be coupled to the cell reaching a predetermined high voltage, decreasing to a predetermined low voltage or an increase in the cell's temperature. This is in contrast with the lithium ion cell which requires a two step charging process to achieve optimum performance. The first step of this process provides that the battery charger apply a constant current level while the cell's voltage remains below a predetermined threshold. Once the voltage increases to that threshold, the second step insures the battery charger is held at the threshold voltage allowing the current to decrease. Once the current decreases sufficiently to a desired level, the lithium ion cell is fully recharged.
This two step process presents a problem when considering charging lithium ion cells in a charger designed for nickel systems. Generally, nickel system chargers apply only a constant current which allows the voltage of the cells to rise unimpeded. The voltage may rise to any level provided the battery pack does not become too hot, i.e. increase to a undesired and dangerous level. Once the nickel system battery pack becomes hot, the charger detects this state and switches from the rapid high current charge to a value of approximately 5-10% that of the rapid current value. This lower current mode is generally referred to as a trickle current or trickle charge.
Hence, the charging scheme offered by current nickel system chargers will not properly charge a lithium ion cell. In particular, if a lithium ion type cell or battery is placed or forced into a nickel system charger, the nickel system charger can cause the voltage of the lithium ion type cell to exceed the threshold voltage of the cell. Therefore, to protect lithium ion type cells, an overvoltage based disconnect circuit should be placed in a lithium ion type battery pack. Such a circuit disconnects the battery pack from the charger if the voltage of the cell or cells reaches the threshold level.
A disconnect circuit for a lithium ion battery pack must include hysteresis for the voltage threshold. Otherwise when the circuit disconnects the battery pack from the charger, and therefore removes current from the cell or cells, the voltage of the cells will drop below the threshold, and the disconnect circuit will reconnect the battery pack to the charger. This may result in a continuous oscillation mode cycling between connect and disconnect modes. During these oscillations, it is very likely that transients may be generated which will cause the cell voltage to exceed the threshold level. Providing hysteresis in the disconnect circuit will accomodate the voltage drop of a cell when the current is removed, and keep the battery pack disconnected until it is removed from the charger.
Therefore there is a need, in a lithium ion battery pack which may be recharged in a nickel system charger, for an overvoltage disconnect circuit which operates according to a predefined hysteresis.